# Control Dielectric vs. Controlled Impedance

Can you tell me what the differences are between Controlled Dielectric and Controlled Impedance. Also what is a TDR report?

I'd appreciate it.

• TDR - Time Domain Reflectometry. Basically it's a measure of the reflections and spurious signals in a conductor arising from the source coupling into non-ideal impedances (e.g. the connecting wiring and termination). It's a common tool used for very high speed digital and analog work. – Connor Wolf Sep 15 '11 at 0:01
• @FakeName: That should be an answer – endolith Sep 15 '11 at 2:35
• @endolith - Well, it only really answers half the question, but ok. – Connor Wolf Sep 15 '11 at 3:25
• @FakeName: If two people answer two halves of the question, then the question is wholly answered! – endolith Sep 15 '11 at 4:29
• @endolith - yes, but that breaks the "accept an answer" system (You cannot accept two answers). I'd rather leave half as a comment, which someone else can put in their other-half answer and make one complete answer. – Connor Wolf Sep 15 '11 at 22:05

Controlled impedance implies that the the PCB is manufactured so that the impedance of traces are within a certain percentage of a specified value. Controlled Dielectric merely specifies that the dielectric constant of the PCB substrate is within a certain percentage of a specific value.

A controlled impedance board generally also has a controlled dielectric substrate, but the inverse is not necessarily true.

Advanced Circuits has a nice short overview here, and reproduced below.

Impedance is the sum of the resistance and reactance of an electrical circuit expressed in Ohms. The resistance being the opposition to current flow present in all materials. The reactance is the opposition to current flow resulting from the effect of the inherent capacitance and inductance of the conductor interacting with changes in voltage and current. In DC circuits there is no reactance and the resistance of copper conductors is typically insignificant. However in high speed AC circuits (those with sharp changes in voltage and/or current) the reactance and thus the impedance can become very significant. This can become critical to a design's functionality because of the effects that changes in the impedance along the signals path from transmitter to receiver will have on the efficiency of power transfer as well as signal integrity. While a circuit’s speed is often expressed as the frequency of the wave form: the critical concern is the speed at which the voltage and/or current is required to change.

Controlled Delectric
The typical design considerations involved in the determination of the requirement for controlling the impedance are the strength of the signals involved, the susceptibility of the circuit to noise and signal distortion, the criticality of signal timing and the speed at which the signal’s source is attempting to force a change in voltage and/or current.

The design considerations for setting the value of the impedance for the conductor is typically the output impedance of the transmitter and the input impedance of the receiver. The impedance of other conductors (i.e. coaxial cables) in the circuit path will also need to be considered. The acceptable range (tolerance) for the impedance will need to be determined and taken into account during the design phase as well as when specifying the PCB parameters. In many cases merely by using software models to determine the anticipated impedance with particular dielectric materials and spacing, followed by requesting these parameters be followed when the PCB’s are fabricated will suffice. This is what we call “controlled dielectric”. For more critical applications you will need to specify “controlled impedance” and supply the actual impedance requirements for the conductors and we will fine tune the dielectrics and conductors to meet these requirements. The impedance requirements will have to be specified based on the layer and the conductor widths.

TDR is Time Domain Reflectometry. Basically it's a measure of the reflections and spurious signals in a conductor arising from the source coupling into non-ideal impedances (e.g. the connecting wiring and termination). It's a common tool used for very high speed digital and analog work.

Therefore, a TDR report is generally either a simulation of a circuit board which tries to account from all of the coupling of a trace into nearby traces and grounds, together with the input and output buffers and terminations of two (or more) chips connected by PCB traces, or an actual text of the circuit-board after it has been manufactured.